Concrete manufacturing contributes to more than 8% anthropogenic CO₂ emission every year. Singapore is a tropical island country where the degradation of construction material is rapid. It is also short of natural resources for construction, and land area for waste deposition. An integrated solution to all these problems is proposed by our team at NUS. We research and innovate the protocols to use the local waste, such as marine clay and municipal waste ash to replace natural constituents for concreting. Both performance and long-term serviceability of our sustainable concrete are proven comparable or even better than conventional concrete

Concrete is vital to modern construction. It is consumed in such huge quantities that it accounts for ~8% anthropogenic CO₂ emission, which mainly comes from the manufacturing of cement. The construction practice in Singapore yearly produces 8-10 million m³ of soft marine clay. Deposition of these clay materials demands a large land area. Meanwhile, other types of solid waste such as incinerated municipal waste ash are also generated in gigantic amounts and end up being landfilled in the offshore Semakau Island, which is projected to run out of capacity by 2035.

Recently, via replacing as much as 50% PC with limestone and clay (calcined to only 700-800°C), a novel cementitious system named ‘Limestone Calcined Clay Cement’ (LC3) has been reported. It exhibits a comparable performance as PC, while reducing the net CO₂ emission by as much as 40%. This new cementitious recipe has inspired us to develop a sustainable concrete using the above mentioned solid waste. The marine clay could provide the reactive kaolinitic content, and other solid waste may provide the carbonate contents. The developed cementitious system is proven to have an extremely low permeability, which in turn provides a robust sealing for the harmful content in the solid wastes.

Experiments were conducted to investigate the performance of the sustainable cementitious system, particularly their durability in marine condition. Mortar cube specimens were made and exposed to simulated seawater for several months. After three months, chloride penetrates to a depth of ~25 mm in ordinary cement system, but only ~7.5 mm in the sustainable cement system (Figure 1a). Electrochemical measurement indicates that the sustainable cement_1 is more resistant to rebar corrosion, as it takes three times the accelerated exposure time for the corrosion potential to drop to the corrosion-risk region, compared with ordinary cement, and other sustainable cement system which has an over-substitution of OPC content.

 Figure 1. (a) Chloride ingress profile after 91 days of exposure. (b) Open circuit potential as a function of exposure time 

Strength is also a key index to evaluate the performance. Figure 2 is an example of the early strength development of several sustainable cement systems. In particular, several systems contain up to 20% dosage of the municipal incineration fly ash (IFA). At 7 days, the strength of all systems are comparable. IFA seems to be more compatible with LC3 compared with OPC, indicating that the synergetic treatment of marine clay and incineration ash has extra benefits for the strength.

Figure 2. Strength development of mortar samples (w/c =0.4) made with ordinary cement (OPC), sustainable cement (lC3) and Municipal incineration fly ash (IFA). A and B denotes the LC3 samples with IFA untreated (A) and pre-washed (B)

The project has the following impact on the construction industry and society. It provides a solution to turn local solid waste into valuable construction materials. The novel cementitious system may elongate the service life of construction materials in marine conditions. Lastly, it contributes to Singapore’s plan of a low-carbon city development, via reducing the consumption of ordinary cement which has a heavy carbon footprint

 

For more details, please contact:
Dr Geng Guoqing
Email: ceegg@nus.edu.sg